Field of the Invention
The invention relates to a method for generating magnetic resonance image data of a subject to be examined by operation of a magnetic resonance scanner, wherein the subject to be examined is transported on an apparatus table or a patient table relative to a magnet/gradient system of the magnetic resonance scanner during a magnetic resonance measurement and raw image data are acquired. Image data of the slices can then be reconstructed on the basis of said raw image data. The invention further relates to a magnetic resonance tomography system having a magnetic resonance scanner, as well as a control device in order to carry out such a method.
Description of the Prior Art
In a magnetic resonance system, the body to be examined is normally subjected to a relatively high constant magnetic field, the so-called “B0 field”, for example of 3 or 7 Tesla, with the use of a basic field magnet system. In addition, a magnetic field gradient is created by a gradient system. Radio-frequency excitation signals (RF pulses), the so-called “B1 field”, are then emitted by a radio-frequency transmitter system having suitable antenna facilities, which is intended to cause the nuclear spins of particular atoms resonantly excited by the radio-frequency field to be flipped (deflected) in a spatially resolved fashion by a defined flip angle with respect to the magnetic field lines of the constant magnetic field. When relaxation of the nuclear spins occurs, radio-frequency signals, so-called magnetic resonance signals, are emitted that are received by suitable reception antennas, and are then processed further. In this situation the data acquisition takes place, for example, line by line in the spatial frequency domain, known as “k-space”. On the basis of this raw data, a reconstruction of the image data which represents a reproduction of the interior of the subject to be examined in the “real” spatial domain, then takes place using a Fourier transform.
Earlier MR systems use the same coil as the transmit coil and reception coil, namely a so-called “volume coil” or “body coil” permanently installed in the scanner. A typical structure of a volume coil is a cage-type antenna (birdcage antenna) composed of multiple rods that proceed parallel to the longitudinal axis around a patient chamber of the scanner in which a patient is situated during the examination. At each end, the antenna rods are circularly connected with a capacitor between each pair of adjacent rods. Conventionally, the volume coil is used only as a transmit coil during the radio-frequency irradiation in order to produce as homogeneous a B1 field as possible perpendicular to the direction of the constant magnetic field. Signal reception however takes place most often using a dedicated reception coil, usually referred to as a “local coil”, which is placed as close as possible to the organ to be examined of the patient.
Measurements with a table traveling continuously through the magnet of the magnetic resonance scanner serve to extend the field of view in the direction of the table displacement (FOVz) and simultaneously to restrict the measuring range inside the magnet, for example to a small region around the isocenter of the magnetic resonance scanner, which is the location of maximum homogeneity of the magnetic field and maximum linearity of the gradient system. A technique competing with continuous table feed is the acquisition of the FOV extended in the table feed direction in a number of stations with the table at a standstill in each case. In this case, after all the data from one station has been acquired, the patient is moved by the patient table to the next station, and the measurement is paused during this movement.
Conventionally, sequences having a very short repetition time (usually, and in the following, denoted as TR) are principally used in the case of acquisition techniques employing continuous feeding of the patient table.
These include, for example, sequences such as TrueFISP (“True Fast Imaging with Steady state Precession”) or proton density-weighted FLASH (“Fast Low Angle Shot”) sequences. In the case of sequences having a very short repetition time, it is possible to consecutively (successively) acquire the raw image data for an individual slice in the center the magnet while the patient (or, more generally, the subject to be examined) is being moved at constant speed
                              V          table                =                  d                                    N              exc                        ⁢            TR                                              (        1        )            through the system. In this case TR denotes the time between the successive excitation of a slice and Nexc denotes the number of excitations per slice which is needed in order to acquire the raw image data for coding an image. When using a Cartesian acquisition technique, in the simplest case (without using parallel acquisition techniques) Nexc is for example equal to the number of phase-encoding steps per slice. When using a radial acquisition technique, in the simplest case (one spoke per excitation) Nexc is equal to the number of spokes measured per image. In the formula (1), d is the distance between adjacent slices (measured from center to center).
In the case of this successive acquisition technique, the data for a first slice are acquired completely before the data acquisition for a further slice commences.
Furthermore, a technique using continuous table feed has become available in which sequences having a moderate TR (such as for example in the case of T1-weighted imaging with FLASH) or a long TR (such as for example in the case of T2-weighted turbo spin echo sequences) are also employed. Since the classical scanning technique using continuous table feed (in other words the successive acquisition of each slice just as it passes through the isocenter of the system) would as a consequence of the long repetition time TR result in this case in an extremely slow table speed and thus to an extremely long examination time (and correspondingly low efficiency), an interleaving technique is generally employed, wherein the data for each slice are acquired at different positions within the magnetic resonance tomograph. This acquisition technique increases the efficiency compared with the classical acquisition technique but also has the actual advantages of the techniques having continuous table feed, namely the acquisition of all slices and data close to the isocenter. Rather, the interleaving results in different data from one and the same slice (in the subject to be examined or patient) necessarily being acquired at different locations within the magnetic resonance scanner. This is a new possible source of artifacts because different scanning conditions exist at different locations within the magnetic resonance scanner due to the imperfect homogeneity of the magnetic field and the imperfect linearity of the gradient system.